Medical system including a novel bipolar pacing pair

ABSTRACT

An implantable electrical medical system comprises a low voltage cathode electrode assembly, including a cathode surface adapted for intimate contact with electrically active tissue, and a low voltage anode electrode assembly, including an anode surface and a porous layer extending over the anode surface. The cathode surface and the anode surface function as a bipolar pair for pacing and the porous layer extending over the anode surface allows conduction therethrough and prevents the anode surface from contacting the electrically active tissue in order to prevent anodal stimulation.

[0001] This application is a continuation-in-part of application Ser.No. 10/439,201, filed May 15, 2003.

FIELD OF THE INVENTION

[0002] Embodiments of the present invention generally relate to thefield of cardiac pacing and/or defibrillation, and more particularly toenhanced left heart pacing.

BACKGROUND

[0003] In the field of cardiac pacing and/or defibrillation, therapydelivery from an implanted medical device may rely upon cardiac signalssensed and pacing therapy delivered via a bipolar pair of implantedelectrodes included on one or more medical electrical leads coupled tothe medical device.

[0004] With respect to sensing, accurate detection and classification ofarrhythmias relies upon an adequate signal-to-noise ratio picked up bythe bipolar pair of electrodes; the signal being a near-field cardiacconduction signal and the noise being either a far-field cardiacconduction signal or electrical activity in other muscles of the body ora combination thereof. Many medical devices incorporate sensingalgorithms to blank or ignore far-field signals, however this may leadto under-sensing or under-detection of fast regular rhythms. As analternative, a spacing between the bipolar pair of electrodes on thelead may be decreased in order reduce and localize the field of sensingbetween the two electrodes.

[0005] With respect to pacing, an effective stimulating pulse is focusedvia intimate tissue contact with a first electrode, serving as acathode, included in the bipolar pair; if a second electrode of thebipolar pair, serving as an anode, comes too close to active tissuethere is a chance of anodal stimulation, which impairs the therapydelivery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The following drawings are illustrative of particular embodimentsof the invention and therefore do not limit the scope of the invention,but are presented to assist in providing a proper understanding. Thedrawings are not to scale (unless so stated) and are intended for use inconjunction with the explanations in the following detailed description.The present invention will hereinafter be described in conjunction withthe appended drawings, wherein like numerals and letters denote likeelements, and:

[0007]FIG. 1 is a schematic of a medical system according to oneembodiment of the present invention;

[0008]FIG. 2A is a plan view of a medical electrical lead according toone embodiment of the present invention;

[0009]FIG. 2B is a plan view of a distal portion of a medical electricallead according to another embodiment of the present invention;

[0010]FIG. 2C is a plan view of a distal portion of a medical electricallead according to yet another embodiment of the present invention;

[0011] FIGS. 3A-C are enlarged schematic plan views of portions ofporous layers according to alternate embodiments of the presentinvention;

[0012]FIG. 4A is a schematic of a lead system implanted within a rightside of a heart according to one embodiment of the present invention;

[0013]FIG. 4B is a series of signal traces illustrating functionaccording to embodiments of the present invention;

[0014]FIG. 5 is a graph illustrating results of a twelve-week animalstudy evaluating embodiments of the present invention; and

[0015]FIG. 6 is a schematic representation of a medical system accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0016]FIG. 1 is a schematic of a medical system according to oneembodiment of the present invention. FIG. 1 illustrates the medicalsystem including an implantable medical electrical lead 2 coupled animplantable medical device (IMD) 1 via a connector header 15; connectorheader 15 includes a bore 16 to receive a connector formed at a proximalend 21 of lead 2 wherein electrical contacts 17 and 18 of header 15couple with lead contacts 22 and 23 of connector, respectively. Header15 is attached to a hermetically sealed enclosure 10 that contains abattery, electronic circuitry and other components known to thoseskilled in the art, and electrical contacts 17 and 18 are any type knownto those skilled in the art that are electrically connected viafeedthroughs (not shown) mounted to extend through hermetically sealedenclosure 10 in order to electrically couple lead 2 with IMD 1.

[0017]FIG. 1 further illustrates lead 2 including first electrode 25joined to a lead body 30 in proximity to a distal end 24 and a secondelectrode 25 joined to lead body 30 in proximity to first electrode 26and spaced a distance X from first electrode 26. First electrode 26 andsecond electrode 25 are electrically coupled to lead contacts 22 and 23via insulated conductors (not shown) extending along lead body 30.According to embodiments of the present invention, first electrode 26and second electrode 25 form a bipolar pair, each having a surface areaadapted for low voltage pacing and sensing, and distance X between firstelectrode 26 and second electrode 25 is less than approximately 9millimeters; furthermore, first electrode 26 has a negative polarity andis adapted for intimate contact with tissue at an implant site andsecond electrode 25 has a positive polarity and is prevented from havingdirect touching contact with tissue adjacent to the implant site by aporous layer (FIGS. 2-3) formed over second electrode 25. Electrodes 25and 26, and other electrodes described herein, according to someembodiments, are comprised of a platinum-iridium alloy. First electrode26, and other first electrodes described herein, may have a poroussurface structure enhancing intimate tissue contact as well as a steroidcoating formed thereover or a plug comprising steroid formed therein.Details associated with electrode fabrication and alternate electrodematerials, including, but not limited to titanium, tantalum, ruthenium,and carbon, are well known to those skilled in the art of leadconstruction. It should be noted that positions of first electrode 26and second electrode 25 might be switched according to alternateembodiments of the present invention.

[0018]FIG. 2A is a plan view including a cut-away section of a medicalelectrical lead 20 according to one embodiment of the present invention,which may be coupled to an IMD just as lead 2 is coupled to IMD 1illustrated in FIG. 1. Examples of IMD's suitable for operation inaccordance with embodiments of the present invention include, but arenot limited to the following Medtronic products: GEM, Marquis DR, JewelAF, AT 500, and Kappa 900.

[0019]FIG. 2A illustrates a body 300 of lead 20 including a distal end240 and a connector 50 formed at a proximal end 210; a first electrode260, joined to distal end 240, is coupled to a first contact 220 ofconnector 50 via a cable conductor 3 extending within insulating sheath5, and a second electrode 251, joined to lead body 300 in proximity tofirst electrode 260 and having a porous layer 252 formed thereover, iscoupled to a second contact 230 via a coil conductor 6. Coupling ofconductors 3 and 6 to first electrode 260 and second electrode 251 andto first contact 220 and second contact 230 may be accomplished by meansof welds or crimps known to those skilled in the art; FIG. 2Aillustrates cable conductor 3 coupled to first electrode 260 via acoupling component 4 wherein cable may be crimped and electrode 260 maybe welded.

[0020] According to embodiments of the present invention, a bipolar pairfor pacing and sensing is formed by first electrode 260 functioning as acathode and second electrode 251 functioning as an anode; layer 252 oversecond electrode 251 allows conduction therethrough while preventingdirect touching contact of electrode 251 with tissue adjacent to animplant site, into which electrode 260 would be fixed. FIG. 2A furtherillustrates first electrode 260 formed as a helix for fixation to theimplant site, however, according to other embodiments a first electrodemay be formed around or within a fixation helix for example along asurface 262 circumscribing helix or on a surface 261 formed in center ofhelix wherein fixation of helix into an implant site will bring surfaces261, 262 into intimate contact with tissue at the implant site.According to one embodiment, second electrode 251 is recessed so that anouter surface 201 of layer 252 is isodiametric with an outer surface 200of lead body 300, as illustrated in FIG. 2A. FIG. 2B is a plan viewincluding a cut-away section of a distal portion of a lead 70illustrating an alternate embodiment wherein an outer surface 202 of aporous layer 254 formed over a second electrode 253 has a diametergreater than outer surface 200 of lead body 300.

[0021]FIG. 2C is a plan view of a distal portion of a lead 80 accordingto yet another embodiment of the present invention. FIG. 2C illustratesa first electrode 860 formed as a generally hemispherical dome and asecond electrode 853 (shown by dashed lines) joined to lead body 300 inproximity to first electrode 860 and having a porous layer 854 formedthereover. FIG. 2C further illustrates a tine structure 880 formed aboutfirst electrode 860 in order to maintain intimate contact of firstelectrode 860 with tissue at an implant site.

[0022] According to embodiments of the present invention a maximumthickness of a porous layer covering a sensing anode, such as layers252, 254, 854, is between approximately 0.005 inch and approximately0.020 inch, and pore sizes of the layer, on average are betweenapproximately 0.4 micron and approximately 50 microns. FIGS. 3A-C areenlarged schematic plan views of portions of various porous layersaccording to alternate embodiments of the present invention. FIGS. 3Aand 3B illustrates pores 91 and 92, respectively formed in asubstantially uniform pattern, while FIG. 3C illustrates pores 95 formedin a substantially random manner. According to one embodiment of thepresent invention a porous layer may be formed of a polymer, such assilicone or polyurethane, wherein pores, e.g. 91, are holes formed bymechanical means, e.g. drilling, or thermal means, e.g. laserperforation, as generally illustrated in FIG. 3A. According to analternate embodiment of the present invention, a porous layer may beformed of a polymer material having a porous microstructure such asexpanded polytetrafluoroethylene (e-PTFE), as is generally illustratedin FIG. 3B wherein pores 92 are formed by a network of fibrils 93connected at nodes 94. Furthermore, an alternate embodiment of a porouslayer may employ a sheet of collagen as illustrated in FIG. 3C, whereinpores 95 are formed by a network of collagen fibers 96. FIGS. 3A-Cpresent exemplary porous layers; according to the present invention anytype of porous material, which is biocompatible and physically separatesa low voltage sensing anode (e.g. second electrodes 251, 253, 853), of abipolar pair, from tissue in proximity to an implant site while allowingadequate electrical conduction therethrough is in accordance with thespirit of the present invention. In a subset of embodiments a porouslayer includes pore sizes, on average, ranging between approximately 0.4microns and approximately 20 microns in order to prevent chronic tissueingrowth. Some embodiments of the present invention wherein a porouslayer is hydrophobic, e.g. e-PTFE, include a wetting agent impregnatedwithin or spread over the porous layer in order to facilitate passage offluid through the porous layer necessary for electrical conduction.Examples of wetting agents include surfactants, hydrogels, gelatins orcombinations thereof; the use of two surfactants, sodium dioctylsulfosuccinate (DSS) and tridodecylmethylammonium chloride (TDMAC), inconjunction with e-PTFE is taught by Carson in U.S. Pat. No. 5,931,862,the teachings from which are incorporated herein. Alternate embodimentsemploy porous layers, surfaces of which are treated to enhancewettability; examples of treatments include but are not limited toplasma processes.

[0023]FIG. 4A is a schematic of a lead system implanted within a rightside of a heart according to one embodiment of the present invention.FIG. 4A illustrates an atrial lead 350 implanted within an atrium 60 bymeans of a first electrode 356 fixed to an atrial appendage implant site62, and a ventricular lead 7 implanted within a ventricle 65 with a tipelectrode 13 at an apical implant site 162. According to embodiments ofthe present invention, leads 350 and 7 are coupled to an IMD, such asany of the aforementioned exemplary IMD's, to form a dual chamber systemwherein a second electrode 355 of atrial lead 350, positioned in closeproximity to first electrode 356, for example spaced a distance X (FIGS.1 and 2) from first electrode 356, includes a porous layer formedthereover (FIGS. 2 and 3) to prevent direct touching contact withcardiac tissue along wall 63 adjacent to implant site 62. Firstelectrode 356 and second electrode 355 form a bipolar pair, firstelectrode 356 being a cathode and second electrode 355 being an anode,for pacing and sensing, wherein sensing of near field cardiac signals,or P-waves, is enhanced as illustrated in panel C of FIG. 4B. Accordingto embodiments of the present invention, enhanced sensing of near fieldsignals improves detection and classification of arrhythmias fordelivery of appropriate therapy via lead 350 and/or lead 7 from an IMD.In some embodiments according to the present invention, a secondelectrode 12 of lead 7 also includes a porous layer formed thereover forenhanced sensing of near-field signals within ventricle 65. As furtherillustrated in FIG. 4A, leads 350 and 7 may also include defibrillationelectrodes 29 and 14, respectively, shown with dashed lines, fordelivery of high voltage stimulation.

[0024]FIG. 4B is a series of signal traces, shown in three panels, A, Band C. In each panel, the top traces, A1, B1 and C1 represent near-fieldsignals sensed by a bipolar pair, while the bottom traces, A2, A3, B2,B3 and C2, C3 represent unipolar components of each top trace: A2, B2and C2 are signals from a first electrode in intimate contact withtissue at an implant site, and A3, B3, and C3 are signals from a secondelectrode spaced proximally from the first electrode. Signal trace A1illustrates sensing by a bipolar pair of electrodes spaced approximately9 millimeters apart, signal trace B1 illustrates sensing by a bipolarpair of electrodes spaced approximately 4 millimeters apart and signaltrace C1 illustrates sensing by a bipolar pair of electrodes spacedapproximately 4 millimeters apart wherein the second electrode includesa porous layer formed thereover to prevent direct touching contact withtissue adjacent to the implant site, for example second electrode 355illustrated in FIG. 4A, according to an embodiment of the presentinvention. As illustrated in FIG. 4B, signal trace C3 has a higheramplitude and slew rate than signal trace A3 but a lower amplitude andslew rate than signal trace B3, which is almost identical to B2;therefore, a combination of closer spacing between electrodes and aporous layer separating second electrode from direct touching contactwith tissue adjacent to an implant site results in the largestpeak-to-peak amplitude of the bipolar signal, C1, illustrated in FIG.4B. Furthermore, according to embodiments of the present invention,prevention of direct touching contact between an anode, for examplesecond electrodes 355 and 12 illustrated in FIG. 4A, and electricallyactive tissue via a porous layer prevents anodal stimulation of thetissue.

EXAMPLE

[0025] A first type of lead including a ring electrode (anode) having aporous layer formed thereover and spaced 4 millimeters from a helicaltip electrode (cathode) was compared to a second type of lead includinga ring electrode spaced 9 millimeters from a helical tip electrode. Twotypes of porous layers were employed in the first type of lead used inour study: 1.) a layer of polyurethane having a thickness ofapproximately 0.008 inch and a durometer of approximately 80 on a shoreA scale, wherein holes, having on average a diameter of 0.001 inch, wereformed by an excimer laser; and 2.) a layer of e-PTFE, obtained fromZeus (part no. 2E055-010 EO*AC), having a thickness of approximately0.010 inch and including pores having, on average, a size betweenapproximately 10 microns and approximately 20 microns. Both leads wereimplanted in a right atrial appendage of six sheep for 12 weeks.Unfiltered P-wave and far-field R-wave (FFRW) amplitudes were measuredduring sinus rhythm (SR) at implant, and 1, 3, 5, 8, and 12 weeks underisoflurane anesthesia. Atrial fibrillation (AF) was induced with 50 Hzrapid pacing and vagal stimulation at 12 wks, at which time, bipolarelectrograms from both leads were input to an ICD atrial sense amplifier(band pass: 16 to 46 Hz), during the AF to evaluate sensing performance.

[0026] Previous studies have shown that reducing the tip-to-ring spacing(TRS) reduces FFRW oversensing; however, short TRS has been associatedwith reductions in P-wave amplitude due to a close proximity of theanode to tissue adjacent to the implant site resulting in contactbetween the anode and active tissue. The results of our study indicatethat a short TRS is feasible when the anode does not contactelectrically active tissue, being separated by a porous layer. FIG. 5presents a graph illustrating our results in which no difference wasfound between chronic P-wave amplitudes sensed by the second type oflead having a 9 mm TRS and the first type of lead having a 4 mm TRS, andwherein FFRW amplitudes were 50% lower as sensed by the first type oflead having the 4 mm TRS. Furthermore, our study found no difference inpacing thresholds between the second type of lead having the 9 mm TRSand the first type of lead having the 4 mm TRS and the porous layer ofe-PTFE formed over the anode. Finally, no difference in sensingperformance was found between unfiltered and filtered signals obtainedfrom both types of leads compared at week 12.

[0027]FIG. 6 is a schematic representation of a medical system accordingto another embodiment of the present invention, wherein a firstelectrode 656, which forms a bipolar pair with a second electrode 612,is joined to a separate lead 600. FIG. 6 illustrates an exemplary systememployed for dual chamber pacing including a first electrode 656 of afirst lead 600 and a second and third electrode 612, 613 of a secondlead 670 coupled to medical device 1 via lead contact 622 of first lead600 engaged by electrical contact 617 in a header bore 615 and leadcontacts 603 and 623 of second lead 670 engaged by electrical contacts602 and 618, respectively, in a header bore 616. FIG. 6 furtherillustrates first lead 600 implanted in a coronary vein 660 forstimulation and sensing of a heart left side 680 and second lead 670implanted in right ventricle 65 for sensing and stimulation of rightventricle 65; such an arrangement of leads included in a medical systemis well known to those skilled in the art cardiac resynchronizationtherapy. According to an embodiment of the present invention, secondelectrode 612 includes a porous layer formed thereover of any of thepreviously described embodiments (FIGS. 2A-3C); the porous layer allowsconduction therethrough while preventing direct contact of secondelectrode 612 with tissue along a wall 663 adjacent apical implant site162 in order to prevent anodal stimulation of right ventricle 65 whensecond electrode 612 forms a bipolar pair with first electrode 656 forstimulation of heart left side 680. Construction of first and secondleads 600, 670 is similar to that described in conjunction with FIGS.2A-C and materials and methods are well known to those skilled in theart. Furthermore, as previously described, first electrode 656 isadapted for intimate tissue contact and construction details for suchelectrodes are also well known to those skilled in the art.

[0028] While specific embodiments have been presented in the foregoingdetailed description, it should be clear that a vast number ofvariations exist. It should also be appreciated that the exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Forexample, electrodes according to embodiments of the present invention,although illustrated in proximity to a distal end of a lead, may belocated at a position anywhere along a length of an implanted lead.Therefore, the foregoing detailed description provides those skilled inthe art with a convenient road-map for implementing an exemplaryembodiment of the invention. It should be understood that variouschanges may be made to exemplary embodiments without departing from thescope of the invention as set forth in the appended claims.

1. A medical system, comprising: an implantable medical device; a firstelongated lead body including a first elongated insulated conductor anda connector formed at a proximal end; the connector including a firstelectrical contact adapted to be electrically coupled to the implantablemedical device; a second elongated lead body including a secondelongated insulated conductor and a connector formed at a proximal end;the connector including a second electrical contact, the secondelectrical contact of the second connector adapted to be electricallycoupled to the implantable medical device; a first low voltage electrodejoined to the first lead body and coupled to the first contact of thefirst connector via the first conductor, the first electrode adapted forintimate contact with tissue at an implant site; a second low voltageelectrode joined to the second lead body and coupled to the secondcontact of the second connector via the second conductor; and a porouslayer formed over the second electrode, allowing conduction therethroughwhile preventing contact between the second electrode and tissue inproximity to the implant site; wherein, when the first connector and thesecond connector are electrically coupled to the medical device and thefirst electrode is contacting tissue at the implant site, the firstelectrode and the second electrode form a bipolar pair for stimulationof tissue at the implant site.
 2. The medical system of claim 1, whereinthe second electrode includes an outer surface, the porous layerincludes an outer surface, and the second lead body includes an outersurface; the outer surface of the second electrode recessed from theouter surface of the second lead body and the outer surface of theporous layer isodiametric with the outer surface of the second leadbody.
 3. The medical system of claim 1, wherein the porous layercomprises silicone.
 4. The medical system of claim 1, wherein the porouslayer comprises polyurethane.
 5. The medical system of claim 1, whereinthe porous layer comprises expanded PTFE.
 6. The medical system of claim1, wherein the porous layer comprises collagen.
 7. The medical system ofclaim 1, further comprising means to promote wetting of the porouslayer.
 8. The medical system of claim 7, wherein the means to promotewetting comprises a wetting agent applied to the porous layer.
 9. Themedical system of claim 8, wherein the wetting agent comprises asurfactant.
 10. The medical system of claim 7, wherein the means topromote wetting comprises a surface treatment of the porous layer. 11.The medical system of claim 1, wherein the porous layer has a thicknessbetween approximately 0.005 inch and approximately 0.020 inch.
 12. Themedical system of claim 1, wherein the porous layer includes poreshaving sizes ranging, on average, between approximately 0.4 micron andapproximately 50 microns.
 13. The medical system of claim 12, whereinthe pores have sizes ranging, on average, between approximately 0.4micron and approximately 10 microns.
 14. The medical system of claim 12,wherein the pores have sizes ranging, on average, between approximately10 microns and approximately 20 microns.
 15. The medical system of claim12, wherein the pores have sizes ranging, on average, betweenapproximately 20 microns and approximately 50 microns.
 16. The medicalsystem of claim 1, wherein the porous layer is adapted to preventchronic tissue ingrowth.
 17. The medical system of claim 1, wherein thefirst low voltage electrode is implanted in a cardiac vein.
 18. Themedical system of claim 17, wherein the second low voltage electrode isimplanted in a right ventricle.
 19. The medical system of claim 1,further comprising a high voltage electrode and wherein the second leadbody further includes a third insulated conductor and the secondconnector further includes a third electrical contact; the high voltageelectrode joined to the second lead body, isolated from the secondelectrode, adapted for defibrillation stimulation and coupled to thethird electrical contact via the third insulated conductor.
 20. Amedical system, comprising: an implantable medical device; a firstelongated lead body including a first elongated insulated conductor anda connector formed at a proximal end; the connector including a firstelectrical contact adapted to be electrically coupled to the implantablemedical device; a second elongated lead body including a secondelongated insulated conductor and a connector formed at a proximal end;the connector including a second electrical contact, the secondelectrical contact of the second connector adapted to be electricallycoupled to the implantable medical device; a first low voltage electrodejoined to the first lead body and coupled to the first contact of thefirst connector via the first conductor, the first electrode adapted forintimate contact with tissue at an implant site; a second low voltageelectrode joined to the second lead body and coupled to the secondcontact of the second connector via the second conductor; and means forpreventing the second electrode from stimulating tissue in proximity tothe second electrode; wherein, when the first connector and the secondconnector are electrically coupled to the medical device and the firstelectrode is contacting tissue at the implant site, the first electrodeand the second electrode form a bipolar pair for stimulation of tissueat the implant site.
 21. An implantable electrical medical system,comprising: a low voltage cathode electrode assembly including a cathodesurface adapted for intimate contact with electrically active tissue;and a low voltage anode electrode assembly including an anode surfaceand a porous layer extending over the anode surface; wherein the cathodesurface and the anode surface function as a bipolar pair for pacing; andthe porous layer extending over the anode surface allows conductiontherethrough and prevents the anode surface from contacting theelectrically active tissue in order to prevent anodal stimulation. 22.The medical system of claim 21, wherein the porous layer comprisessilicone.
 23. The medical system of claim 21, wherein the porous layercomprises polyurethane.
 24. The medical system of claim 21, wherein theporous layer comprises expanded PTFE.
 25. The medical system of claim21, wherein the porous layer comprises collagen.
 26. The medical systemof claim 21, further comprising means to promote wetting of the porouslayer.
 27. The medical system of claim 26, wherein the means to promotewetting comprises a wetting agent applied to the porous layer.
 28. Themedical system of claim 27, wherein the wetting agent comprises asurfactant.
 29. The medical system of claim 26, wherein the means topromote wetting comprises a surface treatment of the porous layer. 30.The medical system of claim 21, wherein the porous layer has a thicknessbetween approximately 0.005 inch and approximately 0.020 inch.
 31. Themedical system of claim 21, wherein the porous layer includes poreshaving sizes ranging, on average, between approximately 0.4 micron andapproximately 50 microns.
 32. The medical system of claim 21, whereinthe pores have sizes ranging, on average, between approximately 0.4micron and approximately 10 microns.
 33. The medical system of claim 21,wherein the pores have sizes ranging, on average, between approximately10 microns and approximately 20 microns.
 34. The medical system of claim21, wherein the pores have sizes ranging, on average, betweenapproximately 20 microns and approximately 50 microns.
 35. The medicalsystem of claim 21, wherein the porous layer is adapted to preventchronic tissue ingrowth.
 36. The medical system of claim 21, furthercomprising an elongate lead body to which the cathode electrode assemblyand the anode electrode assembly are coupled; wherein the anode surfaceis positioned in close proximity to the cathode surface for bipolarsensing of near-field electrical signals.
 37. The medical system ofclaim 36, wherein a shortest distance between the anode surface and thecathode surface is between approximately 2 millimeters and approximately9 millimeters.
 38. The medical system of claim 37, wherein the shortestdistance is between approximately 2 millimeters and approximately 5millimeters
 39. The medical system of claim 37, wherein the shortestdistance is between approximately 5 millimeters and approximately 9millimeters.
 40. The medical system of claim 36, wherein the cathodeelectrode assembly is positioned distal to the anode electrode assembly.41. The medical system of claim 36, wherein the cathode electrodeassembly includes a helix for fixation of the cathode surface to theactive tissue.
 42. The medical system of claim 36, wherein the anodeelectrode assembly is positioned distal to the cathode electrodeassembly.
 43. The medical system of claim 36, wherein: the anode surfaceis recessed from an outer surface of the lead body; and the porous layerincludes an outer surface approximately isodiametric with the outersurface of the lead body.
 44. The medical system of claim 21, furthercomprising: a first elongate lead body to which the cathode electrodeassembly is coupled; and a second elongate lead body to which the anodeelectrode assembly is coupled; wherein the first elongate lead body isadapted for implantation within a cardiac vein in order that the cathodesurface may contact electrically active tissue of an epicardial surface;and the second elongate lead body is adapted for implantation within acardiac chamber.
 45. The medical system of claim 44, wherein the firstlead body is adapted for implantation such that the cathode surfacecontacts left ventricular electrically active tissue.
 46. The medicalsystem of claim 44, wherein the second lead body is adapted forimplantation in a right ventricular chamber.
 47. The medical system ofclaim 44, wherein: the anode surface is recessed from an outer surfaceof the second lead body; and the porous layer includes an outer surfaceapproximately isodiametric with the outer surface of the second leadbody.